The P-Balloon is a collectible item in the fictional Super Mario universe. It was first seen in Super Mario World. When Mario collects a P-Balloon he swells up like a balloon, allowing him to float across large gaps that would otherwise be uncrossable. The P-Balloon has a set timer, and Mario will start to flash when the timer is ending. The effect appears again in Super Mario 64 DS, but instead of collecting a P-Balloon, you have to get a Power Flower.
The P-Balloon also appeared occasionally in the Super Mario World television cartoon. Notably, in the episode "Mama Luigi", when Luigi gets flattened by a spiked pillar coming down from the ceiling in Bowser's Castle, Mario rolls him up like paper and restores him to normal by placing him right next to a nearby P-Balloon. (In the actual game, Bowser's Castle doesn't have any P-Balloons.)